1. Field of the Invention
The present invention relates to a method and spindle unit for producing a hole or a recess in a work piece which may include flat or curved sheets of fiber-reinforced composite material, metal or combinations thereof.
2. Description of the Related Art
Structures for aerospace and other applications are often made up of thin curved shells of different material such as aluminum, titanium, stainless steel and fiber-reinforced composite materials. In structural applications different components are often fastened by use of bolted joints. Bolt holes for aerospace structures are typically about 4-20 mm diameter with high quality requirements to secure the integrity of the structure.
Hole requirements are related to dimension and damage. Dimension requirements include, for example, cylindrical hole diameter, height of the cylindrical part of the hole, diameter and angle of countersinks, roundness, and alignment with the normal direction of the surface. Damage related requirements include, among other things, allowable burr height, surface finish and, with regard to fiber-reinforced composites, allowable delamination.
Particular problems occur when drilling holes in fiber-reinforced composites. Polymer composite materials have been generally known since the 1950's. These materials are composed of a protective and binding polymer, either a thermoplastic or a thermosetting plastic, usually referred to as the matrix, together with fibers (e.g. glass, carbon or amide fibers), which may be regarded as a reinforcing material. The fibers may be continuous and oriented in specific directions, or they may be relatively short and arranged at random in the matrix. Composites with continuous and oriented fibers give products with mechanical properties superior to those of conventional polymer and metallic materials, especially as far as their weight-related strength and stiffness are concerned. Composites with shorter fibers find an application where rather less demanding properties are called for. One factor obstructing the wider use of composite materials is the absence of effective methods of cutting machining. The physical and chemical properties of the composite materials mean that known machining methods cannot generally be applied with successful results.
Products consisting of composite material often contain holes for various purposes. These holes may be required, for instance, to permit the laying of service lines, assembly or inspection. Bolt holes are a particularly important category of hole. Structures for practical applications are often constructed from components joined together to produce a finished product. The purpose of the joint is to transfer the load from one structural element to another. One common form of joining is the bolted connection, in which the load is transferred by either shearing loads or tensile loads in the bolt. The strength of a bolted connection is influenced to a considerable degree by the quality and precision of the hole. Reference may be made to three particular problem areas when producing holes in polymer based fiber reinforced composite materials:
1. Low interlaminar strength. When machining laminated composite materials, there is a risk of the layers separating (delaminating) because of the low interlaminar strength. Extensive delamination damage can jeopardize the strength of the laminate. PA0 2. Low resistance to heat and cold of certain thermoplastics. The heat generated during machining can cause the matrix to soften and block the tool, making further machining impossible. In order to achieve good hole quality, it is accordingly necessary to provide effective cooling of the tool/hole edge, and for the material removed by cutting (chips, splinters and grinding dust) to be removed continuously from the hole. PA0 3. High wear resistance of fibers. The cutting machining of the fiber composites causes severe wear of the tool because of the good wear characteristics of the fiber materials. This leads to high wear costs, especially when producing holes with a requirement for high precision. PA0 1. The apparatus allows the radial offset between the axis of rotation of the cutting tool and the eccentric axis of rotation to be easily adjusted without the replacement of any parts. PA0 2. The apparatus enables the production of holes of higher precision and quality than can be achieved by using the control system of a robot arm. PA0 3. The method permits the production of holes to tight tolerances. The dimensional accuracy of the hole is determined substantially by the accuracy of positioning the tool relative to a central axis. The requirements imposed on the geometry of the tool are not particularly high, on the other hand, since every individual tool is simply calibrated before use.
The methods used to produce holes in composite laminates are traditional drilling, boring, milling, sawing and grinding. The problem associated with these hole-forming methods as they are applied at the present time is that they are not sufficiently effective for various reasons from a technical/economic point of view.
High wear costs are a general problem associated with cutting machining where high precision is required. Great care must be taken when drilling or boring to ensure that delamination damage is avoided on both the entry and exit sides. Special cutters are required in order to achieve the stipulated hole quality, and special procedures must be formulated. In order to avoid extensive delamination damage on the exit side of the laminate, local lateral pressure must be applied around the edge of the hole. Another previously disclosed method of protecting the exit side from damage is to provide the laminate with an additional protective layer.
Sawing is a distinctly unsuitable method for producing holes with high precision requirements. When producing holes by grinding, use is made of a cylindrically shaped tubular body, the machining end of which is coated with a wear-resistant surface layer. Holes are produced by grinding the surface of the material transversely while first causing the grinding body to rotate. The method is slow and gives low precision.
It should be pointed out in this respect that hole-machining methods, in which a body driven rotatably about an axis of rotation is also caused to execute an orbital motion (i.e., the axis of rotation is displaced in such a way that the side is able to move relative to the edge of the hole), are generally familiar. SE 173 899 discloses a machine tool having a tool carrier rotating eccentrically about a principal axis, in which the distance between the tool carrier and the principal axis is determined by a guide component, which rotates about the principal axis together with the tool carrier. The guide component rotating together with the tool carrier is arranged perpendicular to the principal axis and is executed as a cam capable of rotating about it in relation to the tool holder, with the guiding profile of which cam the tool holder is in direct engagement. The advantages of this invention include, among other things, the absence of free play and the space-saving execution of the guide component. However, the offset between the axis of rotation of the tool holder and the eccentric axis of rotation is fixed and determined by the size of the cam gear. Thus, the offset cannot be easily adjusted without replacing the cam gear within the head. SE 382 506 discloses a rotatably driven, combined cutting tool for making holes in stationary work pieces, which holes can be provided with a conical starting chamfer.
Disclosed in the U.S. Pat. No. 5,641,252 (Eriksson et al.), is a method for machining holes in a fiber-reinforced composite material which presents a significant step forward in the art. The central axis of the hole passes through a predetermined point on the surface of the work piece and is oriented in a certain direction in relation to the longitudinal directions of the fibers in the immediate vicinity of the point. The material is machined simultaneously in both an axial and a radial sense by causing the tool to describe axial motion and rotate not only about its own axis, but also eccentrically about the central axis. This method makes it possible to machine holes without causing delamination in the composite material.
Aerospace and related structures are typically made up of different materials stacked together. Particular problems occur when co-drilling structures including several layers of different materials (material stacks). Such problems include burrs in between the layers, close up holes, and damage in filler material in between layers (liquid shims). Drilling holes using traditional techniques generates heat which may cause rapid wear of the tool. This problem is particularly pronounced when drilling holes in titanium.
It is also known to mount a traditional spindle on a robot arm and to use the control system of the robot to dictate the movements of the cutting tool. A problem is that the precision and quality of the resulting hole is limited by the mechanics of the robot and its associated control system, which are designed for moving and positioning a heavy robot arm. Thus, the precision and performance of the robot mechanics and control system are not sufficient to machine, for example, high precision fastener holes at high speeds using the required motion.